Transparent, flame-retardant, thermoformable, UV-resistant film made from crystallizable thermoplastic, its use, and process for its production

ABSTRACT

The invention concerns transparent, flame retardant, thermoformable, UV-stabilized, single layered or multilayered films, containing a cristallizable thermoplastic material, preferably polyethyleneterephthalate, at least one flame retarding agent and a UV stabilizer as main components. Said films are characterized by good stretchability, thermoformability and good optical and mechanical properties, which makes them and the shaped bodies made from said films suitable for both inner and outer applications.

The invention relates to a transparent, flame-retardant, thermoformable,UV-resistant, oriented film made from crystallizable thermoplastics, thethickness of which is preferably from 10 to 350 μm. The film comprisesat least one flame retardant and one UV absorber and has goodorientability and thermoformability, and has very good optical andmechanical properties, and can be produced cost-effectively. Theinvention further relates to the use of the film and to a process forits production.

BACKGROUND OF THE INVENTION

Transparent, oriented films made from crystallizable thermoplastics withthickness from 10 to 350 μm are well known.

In these films, there is no UV absorber present as light stabilizer andthere is no flame retardant, and the films have insufficientthermoformability, the result being that neither the films nor the itemsand, respectively, moldings produced from them are suitable for indoorand/or outdoor applications where fire protection or, respectively,flame retardancy and thermoformability is demanded. The films do notmeet the requirements for the fire tests to DIN 4102 Part 2 and Part 1or the UL-94 test. Even after a short period in outdoor applications,these films exhibit yellowing and impairment of mechanical propertiesdue to photooxidative degradation by sunlight.

EP-A-0 620 245 describes films whose thermal stability has beenimproved. These films comprise antioxidants suitable for scavenging freeradicals formed within the film and degrading any peroxide formed. Thatspecification does not propose how the UV resistance of these filmsmight be improved. Nor does that specification state whether these filmsare suitable for thermoforming processes.

DE-A 2346 787 describes a flame-retardant polymer which has beenphospholane-modified. Besides the polymer, its use for producing filmsand fibers is also claimed.

The following shortcomings become apparent when thisphospholane-modified polymer is used for film production:

The polymer is very susceptible to hydrolysis and has to be veryeffectively predried. When the polymer is dried using dryers of theprior art it cakes, and production of a film is therefore possible onlyunder very difficult conditions.

The films thus produced under uneconomic conditions embrittle whenexposed to heat, i.e. their mechanical properties are severely impairedby the embrittlement, making the film unusable. This embrittlementoccurs after as little as 48 hours of exposure to heat.

It is an object of the present invention to provide a transparent,flame-retardant, UV-resistant, thermoformable, oriented film withthickness preferably from 10 to 350 μm which is not only producedcost-effectively and has good orientability and good mechanical andoptical properties but also in particular is flame-retardant, does notembrittle on exposure to heat, has good thermoformability, and has highUV resistance.

Flame retardancy means that in what is known as a fire test thetransparent film meets the conditions of DIN 4102 Part 2 and inparticular the conditions of DIN 4102 Part 1 and can be allocated toconstruction materials class B2 and in particular B1 forlow-flammability materials.

It is also intended that the film pass the UL-94 test (Vertical BurningTest for Flammability of Plastic Material), permitting its grading inclass 94 VTM-0. This means that 10 seconds after removal of the Bunsenburner the film has ceased to burn, after 30 seconds no smouldering isobserved, and also no burning drops occur.

High UV resistance means that sunlight or other UV radiation causes no,or only extremely little, damage to the films, and that the films ormolding produced from them are therefore suitable for outdoorapplications and/or critical indoor applications. In particular, after anumber of years in outdoor applications the films are intended not toyellow nor to exhibit any embrittlement or surface cracking, nor to haveany impairment of total mechanical properties. High UV resistancetherefore means that the film absorbs UV light and does not transmitlight until the visible region has been reached.

Thermoformability means that the film can be thermoformed to givecomplex and large-surface-area moldings on commercially availablethermoforming machinery without uneconomic predrying.

Good optical properties include high light transmittance (>80%), highsurface gloss (>100), extremely low haze (<20%), and also low yellownessindex (YI <10).

Good mechanical properties include high modulus of elasticity(E_(MD)>3200 N/mm²; E_(TD)>3500 N/mm²), and also good values for tensilestress at break (in MD>100 N/mm²; in TD>130 N/mm²).

Good orientability includes the capability of the film to give excellentlongitudinal and transverse orientation during its production, withoutbreak-offs.

Cost-effective production includes the capability of the raw materialsor raw material components needed to produce the flame-retardant film tobe dried using familiar industrial dryers of the prior art. It isimportant that the raw materials do not cake and do not become thermallydegraded. These industrial dryers of the prior art include vacuumdryers, fluidized-bed dryers; fixed-bed dryers (tower dryers).

These dryers operate at temperatures of from 100 to 170° C., at whichthe flame-retardant polymers mentioned cake, making film productionimpossible. In the vacuum dryer which provides the mildest dryingconditions, the raw material traverses a range of temperatures fromabout 30 to 130° C. under a vacuum of 50 mbar. After this, what is knownas post-drying is required, in a hopper at temperatures of from 100 to130° C., with a residence time of from 3 to 6 hours. Even here, thepolymer mentioned cakes to an extreme degree.

No embrittlement on exposure to heat means that after 100 hours ofheat-conditioning at 100° C. in a circulating-air oven, the film has notembrittled and does not have disadvantageous mechanical properties.

BRIEF DESCRIPTIONS OF THE INVENTION

The object of the invention is achieved by means of a transparent filmwith a preferred thickness in the range from 10 to 350 μm, whichcomprises a crystallizable thermoplastic as principal constituent,wherein the film comprises at least one UV absorber and at least oneflame retardant. This is a mono or biaxially oriented film.

DETAILED DESCRIPTION OF THE INVENTION

The transparent film comprises a crystallizable thermoplastic asprincipal constituent. According to the invention, crystallizablethermoplastics are crystallizable homopolymers; crystallizablecopolymers; crystallizable compounded materials; crystallizable recycledmaterials, and other types of crystallizable thermoplastics.

Preferred suitable crystallizable or semicrystalline thermoplastics arepolyesters, e.g. polyethylene terephthalate, polybutylene terephthalate,and polyethylene naphthalate, preference being given to polyethyleneterephthalate (PET).

It is important for the invention that the crystallizable thermoplastichas a diethylene glycol content (DEG content) of ≧1.0% by weight,preferably ≧1.2% by weight, in particular ≧1.3% by weight, and/or apolyethylene glycol content (PEG content) of ≧1.0% by weight, preferably≧1.2% by weight, in particular ≧1.3% by weight, and/or an isophthalicacid content (IPA) of from 3 to 10% by weight.

It was more than surprising that by virtue of a higher diethylene glycolcontent and/or polyethylene glycol content and/or IPA content than instandard thermoplastics films can be thermoformed cost-effectively oncommercially available thermoforming plants and provide excellentreproduction of detail.

It is also possible to use mixtures of crystallizable thermoplastics.The crystallinities of the crystallizable thermoplastics are preferablyin the range from 5 to 65%.

The transparent film may be either single-layer or multilayer. The filmmay also have a coating of various copolyesters or adhesion promoters.

According to the invention, the transparent film comprises at least oneUV absorber and at least one flame retardant. The UV absorber isadvantageously fed directly during film production by way of what isknown as masterbatch technology, the concentration of the UV stabilizerpreferably being from 0.01 to 5% by weight, in particular from 0.1 to 4%by weight, based on the weight of the layer of the crystallizablethermoplastic.

According to the invention, the flame retardant is fed directly duringfilm production by way of what is known as masterbatch technology, theconcentration being from 0.5 to 30% by weight, preferably from 1 to 20%by weight, based on the weight of the layer of the crystallizablethermoplastic.

Light, in particular the ultraviolet content of solar radiation, i.e.the wavelength region from 280 to 400 nm, induces degradation inthermoplastics, as a result of which their appearance changes due tocolor change or yellowing, and there is also an adverse effect onmechanical/physical properties.

Inhibition of this photooxidative degradation is of considerableindustrial and economic importance, since otherwise there are drasticlimitations on the applications of many thermoplastics.

The absorption of UV light by polyethylene terephthalates, for example,starts at below 360 nm, increases markedly below 320 nm and is verypronounced at below 300 nm. Maximum absorption occurs at between 280 and300 nm.

In the presence of oxygen it is mainly chain cleavage which occurs, butthere is no crosslinking. The predominant photooxidation products inquantity terms are carbon monoxide, carbon dioxide and carboxylic acids.Besides the direct photolysis of the ester groups, consideration has tobe given to oxidation reactions which likewise form carbon dioxide, viaperoxide radicals.

In the photooxidation of polyethylene terephthalate there can also becleavage of hydrogen at the position α to the ester groups, givinghydroperoxides and decomposition products of these, and this may beaccompanied by chain cleavage (H. Day, D. M. Wiles: J. Appl. Polym. Sci16, 1972, p. 203).

UV stabilizers, i.e. light stabilizers which are UV absorbers, arechemical compounds which can intervene in the physical and chemicalprocesses of light-induced degradation. Carbon black and other pigmentscan give some protection from light. However, these substances areunsuitable for transparent films, since they cause discoloration orcolor change. The only compounds suitable for transparent matt films arethose organic or organometallic compounds which produce no, or onlyextremely slight, color or color change in the thermoplastic to bestabilized, that is to say they are soluble in the thermoplastic.

For the purposes of the present invention, UV stabilizers suitable aslight stabilizers are those which absorb at least 70%, preferably 80%,particularly preferably 90%, of the UV light in the wavelength regionfrom 180 to 380 nm, preferably from 280 to 350 nm. These areparticularly suitable if they are thermally stable in the temperaturerange from 260 to 300° C., that is to say they do not decompose and donot cause release of gases. Examples of UV stabilizers suitable as lightstabilizers are 2-hydroxybenzophenones, 2-hydroxybenzotriazoles,organonickel compounds, salicylic esters, cinnamic ester derivatives,resorcinol monobenzoates, oxanilides, hydroxybenzoic esters, andsterically hindered amines and triazines, and among these preference isgiven to the 2-hydroxybenzotriazoles and the triazines.

The UV stabilizer(s) is (are) preferably present in the outer layer(s).The core layer may also have UV stabilizer, if required.

It was highly surprising that the use of the abovementioned UVstabilizers in films gave the desired result. The skilled worker wouldprobably first have attempted to achieve a certain degree of UVresistance by way of an antioxidant, but would have found that the filmrapidly yellows on weathering.

In the knowledge that UV stabilizers absorb UV light and thereforeprovide protection, the skilled worker would be likely to have usedcommercially available stabilizers. He would then have observed that

-   -   the UV stabilizer has unsatisfactory thermal stability and at        temperatures of from 200 to 240° C. decomposes and releases        gases, and    -   large amounts (from about 10 to 15% by weight) of the UV        stabilizer have to be incorporated so that the UV light is        absorbed and the film therefore not damaged.

At these high concentrations it would have been observed that the filmis already yellow just after it has been produced, with Yellowness Indexdeviations (YI) around 25. It would also have been observed that itsmechanical properties are adversely affected. Orientation would haveproduced exceptional problems, such as

-   -   break-offs due to unsatisfactory strength, i.e. modulus of        elasticity too low,    -   die deposits, causing profile variations,    -   roller deposits from the UV stabilizer, causing impairment of        optical properties (defective adhesion, nonuniform surface), and    -   deposits in stretching frames or heat-setting frames, dropping        onto the film.

It was therefore more than surprising that even low concentrations ofthe UV stabilizer achieve excellent UV protection. It was verysurprising that, together with this excellent UV protection:

-   -   within the accuracy of measurement, the Yellowness Index of the        film is unchanged from that of an unstabilized film;    -   there are no releases of gases, no die deposits and no frame        condensation, and the film therefore has excellent optical        properties and excellent profile and layflat, and    -   the UV-stabilized film has excellent stretchability, and can        therefore be produced in a reliable and stable manner on        high-speed film lines at speeds of up to 420 m/min.

The film of the invention comprises at least one flame retardant, whichis fed by way of what is known as masterbatch technology directly duringproduction of the film, and the amount of flame retardant here is from0.5 to 30.0% by weight, preferably from 1.0 to 20.0% by weight, based onthe weight of the layer of the crystallizable thermoplastic. The ratioof flame retardant to thermoplastic is generally kept at from 60:40 to10.90% by weight during preparation of the masterbatch.

Typical flame retardants include bromine compounds, chloroparaffins andother chlorine compounds, antimony trioxide, and aluminum trihydrates,but the use of the halogen compounds here is disadvantageous due to theoccurrence of halogen-containing byproducts. The low light resistance offilms provided with these materials is moreover a great disadvantage, asis the evolution of hydrogen halides in the event of a fire.

Examples of suitable flame retardants used according to the inventionare organophosphorus compounds, such as carboxyphosphinic acids,anhydrides of these and dimethyl methylphosphonate. A substantive factoraccording to the invention is that the organophosphorus compound issoluble in the thermoplastic, since otherwise the optical propertiesrequired are not complied with.

Since the flame retardants generally have some susceptibility tohydrolysis, the additional use of a hydrolysis stabilizer may bedesirable.

The hydrolysis stabilizers used are generally amounts of from 0.01 to1.0% by weight of phenolic stabilizers, the alkali metal/alkaline earthmetal stearates and/or the alkali metal/alkaline earth metal carbonates.The amounts of phenolic stabilizers used are preferably from 0.05 to0.6% by weight, in particular from 0.15 to 0.3% by weight, and theirmolar mass is preferably above 500 g/mol. Particularly advantageouscompounds are pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

It was more than surprising, therefore, that by using masterbatchtechnology a suitable predrying and/or precrystallization procedure and,if desired, using small amounts of a hydrolysis stabilizer, it ispossible to produce a flame-retardant, thermoformable film with therequired property profile in a cost-effective manner and without anycaking in the dryer, and that on exposure to high temperature the filmdoes not become brittle, and does not break when folded.

Within the accuracy of measurement, there is no adverse effect on theYellowness Index of the film, compared with that of an unmodified film.

With this, the film of the invention is also cost-effective.

It was also very surprising that it is even possible to reuse therecycled material produced from the films or moldings without anyadverse effect on the Yellowness Index of the film.

In its particularly preferred embodiment, the film of the invention alsocomprises from 0.01 to 5.0% by weight of2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol of the formula:

or from 0.01 to 5.0% by weight of2,2-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,2,2-tetramethylpropyl)phenolof the formula:

In a preferred embodiment it is also possible for mixtures of the two UVstabilizers mentioned or mixtures of at least one of the two UVstabilizers with other UV stabilizers to be used, where the totalconcentration of light stabilizer is preferably from 0.01 to 5.0% byweight, based on the weight of crystallizable polyethyleneterephthalate.

These data for flame retardant, UV stabilizer, and hydrolysis stabilizerare also valid for other thermoplastics to be used according to theinvention.

The surface gloss, measured to DIN 67530 (measurement angle: 20°), isabove 100, preferably above 120, the light transmittance L, measured toASTM D 1003, is above 80%, preferably above 84%, and the haze of thefilm, measured to ASTM S 1003, is below 20%, preferably below 15%. Theseare surprisingly good properties for the UV resistance achieved incombination with the low flammability.

The standard viscosity SV (DCA) of the polyethylene terephthalate,measured in dichloroacetic acid to DIN 53728, is from 600 to 1000,preferably from 700 to 900. The crystalline melting point measured byDSC with a heating rate of 10° C./min is preferably in the range from220 to 280° C.

The poly ethyl ene terephthalate (PET) preferably has a diethyleneglycol content (DEG content) and/or poly ethylene glycol content (PEGcontent) greater than 1.3% by weight, in particular greater than 1.5% byweight. In one particularly preferred embodiment, the DEG content and/orPEG content is from 1.6 to 5% by weight.

It is surprising here that oriented PET films can be thermoformed byvirtue of higher diethylene glycol content and/or polyethylene glycolcontent than that of standard polyester.

The thermoforming process generally encompasses the steps of predrying,heating, molding, cooling, demolding, and heat-conditioning.Surprisingly, during the thermoforming process it was found that thefilms of the invention can be thermoformed without predrying. Thisadvantage drastically reduces the costs of the thermoforming processwhen comparison is made with thermoformed polycarbonate films orthermoformed polymethacrylate films, for which predrying times of from10 to 15 hours at temperatures of from 100 to 120° C. are required,depending on thickness.

The film of the invention, preferably a PET film, which comprises atleast one UV stabilizer and one flame retardant and is thermoformable,may be either a single-layer film or else, a multilayer film.

In the multilayer embodiment, the film is composed of at least one corelayer and of at least one outer layer, preference being given inparticular to a three-layer A-B-A or A-B-C structure. The thicknesses ofthe outer layers are preferably from 0.5 to 2 μm.

For this embodiment it is important that the standard viscosity and theDEG content and/or PEG content of the crystallizable thermoplastic, e.g.the polyethylene terephthalate, of the core layer are similar to thoseof the polyethylene terephthalate or the thermoplastic of the outerlayer(s) which is/are adjacent to the core layer.

In one particular embodiment, the outer layers, too, may be composed ofa polyethylene naphthalate homopolymer or of a polyethyleneterephthalate-polyethylene naphthalate copolymer, or of a compoundedmaterial.

In this embodiment, the standard viscosity of the thermoplastics of theouter layers is likewise similar to that of, for example, thepolyethylene terephthalate of the core layer.

In the multilayer embodiment, the UV absorber is preferably present inthe outer layers. If required, UV absorber may also be provided in thecore layer.

In the multilayer embodiment, the flame retardant is preferably presentin the core layer. If required, flame retardant may also be provided inthe outer layers.

In another embodiment, flame retardant and UV absorber may also bepresent in the outer layers. If required and if fire protectionrequirements are stringent, the core layer may also have what is knownas a base level of flame retardant.

Unlike in the single-layer embodiment, the concentration of the flameretardant and of the UV stabilizer here is based on the weight in themodified layer. The concentration ranges are identical with those in thebase layer.

Very surprisingly, weathering tests to the ISO 4892 test specificationusing the Atlas C165 Weather-Ometer have shown that in order to improvethe UV resistance of a three-layer film it is entirely sufficient forthe outer layers of preferred thickness from 0.5 to 2 μm to be providedwith UV stabilizers.

Again surprisingly, fire tests to DIN 4102 Part 1 and Part 2, and alsothe UL-94 test, have shown that the films of the invention comply withthese requirements.

The flame-retardant, UV-resistant, thermoformable, multilayer films,produced by known coextrusion technology, are therefore of greatereconomic interest than monofilms pro vided with UV stabilizer and flameretardant throughout, since less additives are needed for comparableflame retardancy and UV resistance.

At least one side of the film may also have been provided with ascratch-resistant coating, with a copolyester, or with an adhesionpromoter.

Weathering tests have shown that even after from 5 to 7 years(extrapolated from the weathering tests) in outdoor applications thefilms of the invention generally have no increased yellowing, noembrittlement, no loss of surface gloss, no surface cracking, and noimpairment of mechanical properties.

During production of the film of the invention it was moreover foundthat the film can be oriented longitudinally and transversely withoutbreak-offs. Furthermore, no evolution of gas from the UV stabilizer orflame retardant were found during the production process, and this isvery advantageous, since most UV stabilizers and flame retardantsexhibit problematic evolution of gases at extrusion temperatures above260° C., as a result of which they cannot be used.

Surprisingly, compliance of the films of the invention with constructionmaterials class B1 to DIN 4102 Part 1 and with the UL-94 test extends asfar as thicknesses in the range from 5 to 350 μm.

In the production of the film of the invention it was moreover foundthat the flame retardant can be incorporated by way of masterbatchtechnology with suitable predrying or precrystallization of the flameretardant masterbatch, without caking in the dryer, permittingcost-effective film production.

It was more than surprising that a low concentration of an addedhydrolysis stabilizer in the flame retardant masterbatch furtherfacilitates incorporation, so that throughputs and therefore productionrates can be increased without difficulty. In one very specificembodiment, the film also comprises small amounts of a hydrolysisstabilizer in the layers provided with flame retardant.

Measurements have moreover shown that the film of the invention does notembrittle on exposure to heat at 100° C. over a prolonged period. Thisresult is attributed to the synergistic action of appropriateprecrystallization, predrying, masterbatch technology, and provision ofUV stabilizer.

The film may moreover be thermoformed without predrying, and cantherefore be used to produce complex moldings.

Examples of process parameters found for the thermoforming process wereas follows:

Step of process Film of the invention Predrying Not required Temperatureof mold ° C. 100-160 Heating time <5 sec per 10 μm of thickness Filmtemperature during 160-200 shaping ° C. Orientation factor possible1.5-2.0 Reproduction of detail Good Shrinkage % <1.5

The film or, respectively, the molding of the invention can moreover berecycled without difficulty, without pollution of the environment, andwithout loss of mechanical properties, and is therefore suitable for useas short-lived advertising placards, for example, in the construction ofexhibition stands, and for other promotional items, where fireprotection and thermoformability is desired.

An example of a method of producing the film of the invention is theextrusion process on an extrusion line.

According to the invention, the flame retardant is added here, whereappropriate with the hydrolysis stabilizer, by way of masterbatchtechnology. The flame retardant is dispersed in a carrier material.Carrier materials which may be used are the thermoplastic itself, e.g.the polyethylene terephthalate, or else other polymers compatible withthe thermoplastic.

The light stabilizer may advantageously be fed before the materialleaves the thermoplastic polymer producer, or may be metered into theextruder during film production.

It is particularly preferable to add the light stabilizer by way ofmasterbatch technology. The light stabilizer is dispersed in a solidcarrier material. Carrier materials which may be used are certainresins, the thermoplastic itself, e.g. the polyethylene terephthalate,or else other polymers sufficiently compatible with the thermoplastic.

The DEG content and/or PEG content of the polyethylene terephthalate areadvantageously set at the premises of the polymer producer during thepolycondensation process.

In masterbatch technology it is important that the grain size and thebulk density of the masterbatch is similar to the grain size and thebulk density of the thermoplastic, permitting uniform distribution andtherefore also uniform UV resistance.

The films of the invention may be produced by known processes, e.g. froma polyester with, where appropriate, other raw materials, e.g. the UVabsorber and/or other conventional additives in conventional amounts offrom 1.0 to a maximum of 30% by weight, either in the form of a monofilmor else in the form of multilayer, where appropriate coextruded filmswith identical or differently constructed surfaces, where one surfacemay, for example, have been pigmented but no pigment is present at theother surface. Known processes may also have been used to provide one orboth surfaces of the film with a conventional functional coating.

A substantive factor for the invention is that the masterbatch whichcomprises the flame retardant and, if used, the hydrolysis stabilizer,is precrystallized or predried. This predrying includes gradual heatingof the masterbatch at reduced pressure (from 20 to 80 mbar, preferablyfrom 30 to 60 mbar, in particular from 40 to 50 mbar), with agitation,and, if desired, post-drying at a constant, elevated temperature, againat reduced pressure. It is preferable for the masterbatch to be chargedat room temperature from a metering vessel in the desired blend togetherwith the polymers of the base and/or outer layers and, if desired, withother raw material components batchwise into a vacuum dryer in which thetemperature profile moves from 10 to 160° C., preferably from 20 to 150°C., in particular from 30 to 130° C., during the course of the dryingtime or residence time. During the residence time of about 6 hours,preferably 5 hours, in particular 4 hours, the raw material mixture isstirred at from 10 to 70 rpm, preferably from 15 to 65 rpm, inparticular from 20 to 60 rpm. The resultant precrystallized or predriedraw material mixture is post-dried in a downstream vessel, likewiseevacuated, at temperatures of from 90 to 180° C., preferably from 100 to170° C., in particular from 110 to 160° C., for from 2 to 8 hours,preferably from 3 to 7 hours, in particular from 4 to 6 hours.

In the preferred extrusion process for producing a polyester film of theinvention, the molten polyester material is extruded through a slot dieand quenched on a chill roll, in the form of a substantially amorphousprefilm. This amorphous prefilm is then reheated and stretchedlongitudinally and transversely, or transversely and longitudinally, orlongitudinally, transversely and again longitudinally and/ortransversely. In general, the stretching temperatures are from T_(g)+10°C. to T_(g)+60° C. (where T_(g) is the glass transition temperature),the longitudinal stretching ratio is usually from 2 to 6, in particularfrom 3 to 4.5, and the transverse stretching ratio is from 2 to 5, inparticular from 3 to 4.5, and the ratio for any second longitudinal ortransverse stretching carried out is from 1.1 to 5. The firstlongitudinal stretching may, if desired, be carried out simultaneouslywith the transverse stretching (simultaneous stretching). This isfollowed by the heat-setting of the film at oven temperatures of from180 to 260° C., in particular from 220 to 250° C. The film is thencooled and wound up.

The surprising combination of excellent properties makes the film of theinvention and moldings produced therefrom highly suitable for a varietyof different applications, such as interior decoration, for constructingexhibition stands, for exhibition requisites, for displays, forplacards, for protective glazing of machines or vehicles, in thelighting sector, in fitting out shops or stores, or as a promotionalrequisite, laminating material, for greenhouses, roofing systems,exterior cladding, protective coverings, applications in theconstruction sector, illuminated advertizing profiles, blinds, orelectrical applications.

The thermoformability of the film of the invention makes it suitable forthermoforming any desired moldings for indoor or outdoor applications.

The examples below illustrate the invention in more detail.

The following standards and methods are used here when testingindividual properties.

Test Methods

-   DIN=Deutsches Institut für Normung [German Standards Institute]-   ISO=International Organization for Standardization.    DEG Content, PEG Content, and IPA Content

DEG/PEG/IPA content is determined by gas chromatography aftersaponification in methanolic KOH and neutralization with aqueous HCl.

Surface Gloss

Surface gloss is measured with a measurement angle of 20° to DIN 67530.

Light Transmittance

Light transmittance is the ratio of total light transmitted to theamount of incident light.

Light transmittance is measured using “®HAZEGARD plus” test equipment toASTM D 1003.

Haze

Haze is that percentage proportion of the transmitted light whichdeviates by more than 2.5° from the average direction of the incidentlight beam. Clarity is determined at an angle of less than 2.5°.

Haze is measured using “HAZEGARD plus” apparatus to ASTM D 1003.

Surface Defects

Surface defects are determined visually.

Mechanical Properties

Modulus of elasticity, tensile strength at break and elongation at breakare measured longitudinally and transversely to ISO 527-1-2.

SV (DCA) and IV (DVA)

Standard viscosity SV (DCA) is measured by a method based on DIN 53726in dichloroacetic acid.

Intrinsic viscosity (IV) is calculated as follows from standardviscosity (SV)IV(DCA)=6.67·10⁻⁴ SV(DCA)+0.118Fire Performance

Fire performance is determined to DIN 4102, Part 2, constructionmaterials class B2, and to DIN 4102, Part 1, construction materialsclass B1, and also by the UL-94 test.

Weathering (on Both Sides) and UV Resistance

UV resistance is tested as follows to the ISO 4892 test specification

Test equipment: Atlas Ci 65 Weather-Ometer Test conditions: ISO 4892,i.e. artificial weathering Irradiation time: 1000 hours (per side)Irradiation: 0.5 W/m², 340 nm Temperature: 63° C. Relative humidity: 50%Xenon lamp: Internal and external filter made from borosilicateIrradiation cycles: 102 minutes of UV light, then 18 minutes of UV lightwith water spray onto the specimens, then another 102 minutes of UVlight, etc.

Numerical values of <0.6 are negligible and indicate that there is nosignificant color change.

Yellowness Index

Yellowness Index (YI) is the deviation from the colorless condition inthe “yellow” direction and is measured to DIN 6167. Yellowness Indexvalues (YI)<5 are not visible.

In the examples and comparative examples below each of the films is atransparent film of different thickness, produced on the extrusion linedescribed.

Each of the films was first weathered on both sides to the testspecification of ISO 4892 for 1000 hours per side, using an Atlas Ci 65Weather-Ometer, and then tested for mechanical properties, YellownessIndex (YI), surface defects, light transmittance and gloss.

Fire tests to DIN 4102, Part 2 and Part 1, and the UL-94 test, werecarried out on each film.

EXAMPLES Example 1

A transparent film of 50 μm thickness is produced, comprisingpolyethylene terephthalate as principal constituent, 0.2% by weight of®Sylobloc (silicon dioxide) as antiblocking agent, 4% by weight of theorganophosphorus compound as flame retardant and 1.0% by weight of theUV stabilizer 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol(®Tinuvin 1577, from Ciba-Geigy). Tinuvin 1577 has a melting point of149° C. and is thermally stable up to about 330° C.

To obtain homogeneous distribution, 0.2% by weight of Sylobloc isincorporated directly into the polyethylene terephthalate (PET) when thepolymer is prepared.

The polyethylene terephthalate from which the transparent film isproduced has a standard viscosity SV (DCA) of 810, corresponding to anintrinsic viscosity IV (DCA) of 0.658 dl/g. Both DEG content and PEGcontent are 1.6% by weight.

Tinuvin 1577 has a melting point of 149° C. and is thermally stable upto about 330° C.

The UV stabilizer Tinuvin 1577 is fed in the form of a masterbatch. Themasterbatch is composed of 5% by weight of Tinuvin 1577 as activeingredient and 95% by weight of PET having a standard viscosity SC(DCA)=810, corresponding to an intrinsic viscosity IV (DCA) of 0.658dl/g.

The flame retardant is the organophosphorus compound dimethylmethylphosphonate, ®Amgard P 1045 from Albright & Wilson, which issoluble in PET.

The flame retardant is likewise fed in the form of a masterbatch. Themasterbatch is composed of 20% by weight of flame retardant and 80% byweight of PET having a standard viscosity SV (DCA) of 810.

Both of the masterbatches have bulk density of 750 kg/m³.

40% by weight of PET with 0.2% by weight of Sylobloc, 30% by weight ofrecycled PET material, 10% by weight of UV masterbatch and 20% by weightof flame retardant masterbatch are discharged at room temperature fromseparate metering vessels into a vacuum dryer which operates with atemperature profile of from 25 to 130° C. from the time of charging tothe end of the residence time. During the residence time of about 4hours, the mixture of raw materials is agitated at 61 rpm.

The precrystallized and/or predried mixture of raw materials ispost-dried for 4 hours at 140° C. in a downstream hopper, again invacuo. The 50 μm monofilm is then produced by the extrusion processdescribed.

The transparent PET film produced has the following property profile:

Thickness: 50 μm Surface gloss, Side 1: 155 (Measurement angle 20°) Side2: 152 Light transmittance: 91% Haze: 4.0% Surface defects per m²: none(cracks, embrittlement) Longitudinal modulus of elasticity: 3550 N/mm²Transverse modulus of elasticity: 4700 N/mm² Longitudinal tensilestrength at break: 110 N/mm² Transverse tensile strength at break: 190N/mm² Yellowness Index (YI): 3.1

After 200 hours of heat treatment at 100° C. in a circulating-air dryingcabinet there is no change in mechanical properties. The film shows noembrittlement phenomena of any kind.

The film complies with the requirements for construction materialclasses B2 and B1 to DIN 4102 Part 2/Part 1. The film passes the UL-94test.

After in each case 1000 hours of weathering per side with the Atlas Ci65 Weather-Ometer, the PET film has the following properties:

Thickness: 50 μm Surface gloss, Side 1: 148 (Measurement angle 20°) Side2: 146 Light transmittance: 89.9% Haze: 4.2% Surface defects per m²:none (cracks, embrittlement) Longitudinal modulus of elasticity: 3400N/mm² Transverse modulus of elasticity: 4550 N/mm² Longitudinal tensilestrength at break: 102 N/mm² Transverse tensile strength at break: 178N/mm² Yellowness Index (YI): 3.2

Example 2

Coextrusion technology is used to produce a multilayer PET film havingthe layer sequence A-B-A and a thickness of 17 μm, B being the corelayer and A being the outer layers. The core layer has a thickness of 15μm, and each of the two outer layers which cover the core layer has athickness of 1 μm.

The polyethylene terephthalate used for the core layer B is identicalwith the polymer of Example 1 except that it comprises no Sylobloc. Thecore layer comprises 0.2% by weight of hydrolysis stabilizer and 5% byweight of flame retardant. As in Example 1, the hydrolysis stabilizerand the flame retardant are fed in the form of a masterbatch. Themasterbatch is composed of 25% by weight of flame retardant, 1% byweight of hydrolysis stabilizer and 74% by weight of polyethyleneterephthalate. The flame retardant is identical with that used inExample 1. The hydrolysis stabilizer is pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.

The polyethylene terephthalate of the outer layers A is identical withthe polyethylene terephthalate of Example 1, that is to say the outerlayer polymer has 0.2% by weight of Sylobloc. The outer layers compriseno hydrolysis stabilizer and no flame retardant. The outer layersadditionally comprise 1.0% by weight of Tinuvin 1577, and this amountwas incorporated directly at the premises of the polymer producer.

For the core layer, 50% by weight of polyethylene terephthalate, 30% byweight of recycled polyethylene terephthalate material and 20% by weightof the masterbatch described are precrystallized, predried andpost-dried as in Example 1.

The outer layer polymer, which comprises Sylobloc and 1% by weight ofTinuvin 1577, is not subjected to any particular drying. Coextrusiontechnology is used to produce a film having the layer sequence A-B-A,thickness of 17 μm, and the following property profile:

Layer structure: A-B-A Thickness: 17 μm Surface gloss Side 1: 174(Measurement angle 20°) Side 2: 169 Light transmittance: 94.2% Haze:2.1% Surface defects per m²: none (cracks, embrittlement) Longitudinalmodulus of elasticity: 3500 N/mm² Transverse modulus of elasticity: 4150N/mm² Longitudinal tensile strength at break: 120 N/mm² Transversetensile strength at break: 155 N/mm² Yellowness Index (YI): 2.7

After 200 hours of heat treatment at 100° C. in a circulating-air dryingcabinet there is no change in mechanical properties. The film shows noembrittlement phenomena of any kind.

The film complies with the requirements for construction materialclasses B2 and B1 to DIN 4102 Part 2 and Part 1. The film passes the ULtest.

After in each case 1000 hours of weathering per side with the Atlas Ci65 Weather-Ometer the PET film has the following properties:

Thickness: 17 μm Surface gloss Side 1: 168 (Measurement angle 20°) Side2: 160 Light transmittance: 91.6% Haze: 2.9% Surface defects per m²:none (cracks, embrittlement) Longitudinal modulus of elasticity: 3400N/mm² Transverse modulus of elasticity: 4000 N/mm² Longitudinal tensilestrength at break: 105 N/mm² Transverse tensile strength at break: 145N/mm² Yellowness Index (YI): 3.2

Example 3

As in Example 2, an A-B-A film of 20 μm thickness is produced, the corelayer B having a thickness of 16 μm and each outer layer A having athickness of 2 μm.

The core layer B comprises only 5% by weight of the flame retardantmasterbatch of Example 2.

The outer layers are identical with those of Example 2, except that theyadditionally comprise 20% by weight of the flame retardant masterbatch,used in Example 2 only for the core layer.

The polymers and the masterbatches for the core layer and the outerlayers are precrystallized, predried and post-dried as in Example 1.

The multilayer 20 μm film produced using coextrusion technology has thefollowing property profile:

Layer structure: A-B-A Thickness: 20 μm Surface gloss Side 1: 165(Measurement angle 20°) Side 2: 169 Light transmittance: 92.0% Haze:2.5% Surface defects per m²: none (cracks, embrittlement) Longitudinalmodulus of elasticity: 3450 N/mm² Transverse modulus of elasticity: 4000N/mm² Longitudinal tensile strength at break: 125 N/mm² Transversetensile strength at break: 160 N/mm² Yellowness Index (YI): 2.9

After 200 hours of heat treatment at 100° C. in a circulating-air dryingcabinet there is no change in mechanical properties. The film shows noembrittlement phenomena of any kind.

The film complies with the requirements for the construction materialclasses 2 and B1 to DIN 4102 Part 2 and Part 1. The film passes the ULtest.

After in each case 1000 hours of weathering per side with the Atlas Ci65 Weather-Ometer the PET film has the following properties:

Thickness: 20 μm Surface gloss Side 1: 161 (Measurement angle 20°) Side2: 155 Light transmittance: 91.2% Haze: 3.1% Surface defects per m²:none (cracks, embrittlement) Longitudinal modulus of elasticity: 3400N/mm² Transverse modulus of elasticity: 3850 N/mm² Longitudinal tensilestrength at break: 115 N/mm² Transverse tensile strength at break: 145N/mm² Yellowness Index (YI): 3.5Thermoformability

The films from Examples 1 to 3 may be thermoformed to give moldings oncommercially available thermoforming machinery, e.g. from the companyIllig, without predrying. The reproduction of detail in the moldings isexcellent and the surface is uniform.

Comparative Example 1

Example 2 is repeated, except that the film is not provided with UVstabilizers, nor with flame retardant masterbatch, that is to say thefilm comprises no hydrolysis stabilizer, no flame retardant and no UVstabilizer. DEG content is the commercially available 0.7% by weight,and no PEG is present.

The transparent PET film produced has the following property profile:

Thickness: 17 μm Surface gloss Side 1: 175 (Measurement angle 20°) Side2: 165 Light transmittance: 92% Haze: 2.3% Surface defects per m²: noneLongitudinal modulus of elasticity: 4100 N/mm² Transverse modulus ofelasticity: 4800 N/mm² Longitudinal tensile strength at break: 180 N/mm²Transverse tensile strength at break: 210 N/mm² Yellowness Index (YI):2.8

The unmodified film does not meet the requirements of the tests to DIN4102 Part 1 and Part 2, nor those of the UL-94 test.

The film has insufficient thermoformability.

After 1000 hours of weathering per side using the Atlas CIWeather-Ometer, the film exhibits surface cracks and embrittelementphenomena. It is therefore impossible to measure a precise propertyprofile—in particular mechanical properties. The film also shows visibleyellowing.

1. A transparent, thermoformable, oriented film made from acrystallizable thermoplastic polyester or from a mixture of variouscrystallizable thermoplastic polyester as principal constituent and fromat least one flame retardant and from at least one UV stabilizer,wherein the polyester has at least one of either a diethylene glycolcontent or a polyethylene glycol content of from about 1.0 to about 5%by weight, said film exhibiting a modulus of elasticity in the machinedirection of greater than 3200 N/mm² and a modulus of elasticity in thetransverse direction of greater than 3500 N/mm².
 2. The film as claimedin claim 1, wherein the thermoplastic has a crystallinity of from about5 to about 65%.
 3. The film as claimed in claim 1, wherein thethermoplastic comprises a polyester.
 4. The film as claimed in claim 1,wherein the thermoplastic comprises polyethylene terephthalate,polybutylene terephthalate or polyethylene naphthalate.
 5. The film asclaimed in claim 4, wherein the thermoplastic comprises polyethyleneterephthalate.
 6. The film as claimed in claim 5, wherein thepolyethylene terephthalate has a diethylene glycol content or apolyethylene glycol content or a diethylene glycol content and apolyethylene glycol content of more than about 1.3% by weight.
 7. Thefilm as claimed in claim 5, wherein the polyethylene terephthalate has adiethylene glycol content or a polyethylene glycol content or adiethylene glycol content and a polyethylene glycol content of fromabout 1.6 to about 5% by weight.
 8. The film as claimed in claim 5,wherein the polyethylene terephthalate has a standard viscosity SV (DCA)of from about 600 to about
 1000. 9. The film as claimed in claim 1,wherein the flame retardant comprises organophosphorus compounds ormixtures of organophosphorus compounds and the UV stabilizer comprises2-hydroxybenzotriazoles or triazines or mixtures of these UVstabilizers.
 10. The film as claimed in claim 1, wherein the UVstabilizer comprises2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol or2,2′-methylenebis-6-(2H-benzotriazol-2-yl)-4-(1,1,2,2-tetramethylpropyl)phenolor mixtures of these UV stabilizers or mixtures of these UV stabilizerswith others.
 11. The film as claimed in claim 1, wherein theconcentration of the UV stabilizer or UV stabilizes is from about 0.01to about 5% by weight, based on the weight of the crystallizablethermoplastic.
 12. The film as claimed in claim 9, wherein theorganophosphorus compound or the organophosphorus compounds is or aresoluble in the thermoplastic.
 13. The film as claimed in claim 1,wherein the flame retardant used is dimethyl methylphosphonate.
 14. Thefilm as claimed in claim 1, wherein the concentration of the flameretardant or flame retardants is from about 0.5 to about 30% by weight,based on the weight of the crystallizable thermoplastic.
 15. The film asclaimed in claim 1, wherein the film comprises from about 0.1 to about1% by weight of a hydrolysis stabilizer, based on the weight of thecrystallizable thermoplastic.
 16. The film as claimed in claim 1,wherein the hydrolysis stabilizer is pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.17. The film as claimed in claim 1, wherein the film has two or morelayers.
 18. The film as claimed in claim 17, wherein the flame retardantor the UV stabilizer or the flame retardant and the UV stabilizer is orare in present in the outer layer or layers only.
 19. The film asclaimed in claim 1, wherein the film has a thickness of from about 1 tobout 350 μm.
 20. A process for producing a film as claimed in claim 1,which comprises the steps of melting extruding, biaxially orienting andsetting a crystallizable thermoplastic or a mixture made fromcrystallizable thermoplastics in an extruder, together with at least oneflame retardant and at least one UV stabilizer.
 21. The process asclaimed in claim 20, wherein flame retardant and UV stabilizer are addedby way of masterbatch technology.
 22. A method of making a molding whichmethod comprises transforming a film as claimed in claim 1 into amolding.
 23. A molding comprising a film as claimed in claim
 1. 24. Afilm according to claim 1, wherein said polyester comprises a mixture ofboth diethylene glycol and polyethylene glycol, said mixture present inan amount ranging from about 1.3 to about 5% by weight.
 25. A filmaccording to claim 1, wherein said at least one flame retardant isincorporated into said polyester as a precrystallized or predriedmasterbatch, said polyester thus further comprising precrystallized orpredried carrier material.